Flat panel displays or liquid crystal displays (LCDs) are popular display devices for conveying information generated by computers. Reduced weight and size of a flat panel display offer great advantages over cathode ray tube (CRT) displays. High-quality flat panel displays are typically backlighted, that is, a source of illumination is placed behind the LCD layers to facilitate visualization of the image. Flat panel LCD devices are used in many applications including the computer industry, where flat panel LCD units are an excellent display choice for laptop computers and other portable electronic devices. However, because of rapid progress in the technology of flat panel LCDs, these devices find increasing use in other mainstream applications such as desktop computers, high-end graphics computers, and television and other multimedia monitors.
A liquid crystal display typically comprises a pair of plates with driving electrodes and a layer of twist nematic (TN) or supertwist nematic (STN) liquid crystal material confined between these plates. The liquid crystal layer thickness and anisotropy are such that it is capable of rotating polarization of a normally incident ray of light for at least one wavelength in the visible spectrum by about 80-1000 (for TN) or 180-230° (for STN). The device further comprises a rear light-entrance polarizer having a transmission axis oriented in a certain direction, a front light-exit polarizer having a transmission axis oriented in a direction different from the former one, thereby defining a normally white or normally black display, and a backlight system. The device may also comprise a rear retardation film situated between the rear polarizer and the twisted nematic liquid crystal layer, and a front retardation film situated between the front polarizer and the liquid crystal layer.
A picture on the display is formed by thousands of small imaging elements, or “pixels”, which are either “on”, “off”, or “partially on”. An image is displayed usually by applying an electric field to the individual pixels. In the case of a twist nematic (TN) LCD, if a particular pixel is “on”, then the phase and thus the polarization of a linearly polarized light ray will remain unchanged as it passes through the pixel. However, if the pixel is “off”, then the polarization plane of the light ray will be rotated, that is, its phase will be modulated so that its polarization angle is changed by 90°. If the pixel is “partially on”, then the ray polarization axis will be rotated by less than 90°. An “on” pixel can be designated to represent either black or white. If the “on” pixel is designated as black, then the “off” pixel is designated as white, and vice versa. A “partially on” pixel represents a shade of gray. Polarizers are provided on the LCD so that the polarization state of light passing through a pixel is converted into an appropriate amount of transmission (black, white, or gray).
In the case of a supertwist nematic (STN) LCD, the optical effect is due to birefringence so that each of the “on”, “off”, and “partially on” pixels have a characteristic birefringence color. If the “blue mode” is used, the “off” pixel will have a blue color, while the “on” pixel will be cream colored. If the “yellow mode” is used, the “off” pixel will be yellow and the “on” pixel will be blue-gray. A film may be added between the STN LCD and one of its polarizers to neutralize the display color, that is, to convert the color display into a black-and-white display.
The flat panel LCD is typically provided with a backlight system. Preferably, the backlight system radiates at least partly polarized light. The most effective system capable of converting all the nonpolarized incident light flux of backlight system into polarized light with minimum losses is offered by the so-called optical recycling scheme with a reflective polarizer.
The reflective polarizer usually comprises a multilayer structure consisting of alternating anisotropic and isotropic layers, with the refractive index of an isotropic layer being equal to that of one of the anisotropic layers. This structure is capable of transmitting light in one polarization state while reflecting light polarized in the perpendicular direction. In one of such structures, the reflected polarized light passes through a quarter-wave plate, changes the polarization direction, reflects from a mirror, and enters the reflective polarizer again, this time in the first polarization state. The reflective polarizer is placed on the backlight system or on the rear plates of the LCD.
Many naturally occurring crystalline compounds act as birefringent (or reflective) polarizers. For example, calcite (calcium carbonate) crystals possess well-known birefringent properties. However, single crystals are expensive materials and cannot be readily formed into the desired shapes or configurations, which are required for particular applications. In the prior art, birefringent polarizers were fabricated (see, e.g., Makas, U.S. Pat. No. 3,438,691) from plate-like or sheet-like birefringent polymers such as poly(ethylene terephthalate) incorporated into an isotropic polymer matrix.
In many cases, polymers can be oriented by uniaxial stretching so as to align the polymer chains on a molecular level as described by Rogers et al., U.S. Pat. No. 4,525,413. Multilayer optical devices comprising alternating layers of highly birefringent polymers and isotropic polymers with large refractive index mismatch have been also proposed by Rogers et al. However, these devices require the use of specific highly birefringent polymers obeying certain mathematical relationships between their molecular configurations and electron density distributions.
There is a known birefringent interference polarizer in the form of a multilayer sheet or film, which can be fabricated from readily available materials using well-established coextrusion techniques. The layers can be made of alternating birefringent and isotropic materials. In this system, one of the two indices of refraction of the birefringent material substantially matches the index of refraction of the isotropic material in the adjacent layer, or the alternating layers can be made of two different birefringent materials selected so that the lower of the two indices of refraction of one of the materials substantially matches the higher of the two indices of refraction of the other material. When adjacent layers of the latter embodiment are both positively or negatively birefringent, then their optical axes should be perpendicular; when the birefringence signs are opposite, the two optical axes should be parallel. To reach the extremely high efficiency previously mentioned, the layers should have an optical thickness equal to one-quarter of the selected light wavelength.
Also known is an LCD containing a reflective polarizer on the rear panel, which represents a multilayer structure containing anisotropic layers made of the same materials.
Examples of diffusely reflecting polarizing materials are described in U.S. Pat. Nos. 5,783,120 and 5,825,543 and in PCT Patent Application Publication Nos. WO 97/32223, WO 97/32224, WO 97/32225, WO 97/32226, WO 97/32227, and WO 97/32230. Examples of multilayer reflective polarizers are described in U.S. Pat. No. 5,882,774. Examples of cholesteric reflective polarizers are described in EP No. 606,940 and U.S. Pat. No. 5,325,218.
Some promising materials for manufacturing anisotropic films are described in U.S. Pat. Nos. 5,739,296; 6,174,394; and 6,563,640. PCT Patent Application Publication No. WO 02/63660 describes methods for manufacturing such films. This invention provides for an increase in the display brightness and makes it possible to obtain images with spectrally clean colors and to create white, black and color components in the image, which allows increasing the contrast and richness of the image, as well as the viewing (aspect) angle of the display.